Optical materials including glass materials, organic resins or the like generally exhibit a gradually higher refractive index on a shorter wavelength side. The indexes to indicate the wavelength dispersion of the refractive index include the Abbe number (νd) and the secondary dispersion property (θg,F). The Abbe numbers and the θg,F values are values peculiar to respective optical materials, but fall in certain ranges in many cases. FIG. 1 is a diagram illustrating the relationship between the secondary dispersion properties and the Abbe numbers of conventional optical materials (glass materials and organic resins). Here, the Abbe number (νd) and the secondary dispersion property (θg,F) are represented by the following expressions. Abbe number [νd]=(nd−1)/(nF−nc), and Secondary dispersion property [θg,F]=(ng−nF)/(nF−nc), where nd is a refractive index at a wavelength of 587.6 nm; nF is a refractive index at a wavelength of 486.1 nm; nc is a refractive index at a wavelength of 656.3 nm; and ng is a refractive index at a wavelength of 435.8 nm.
In dioptric systems, a suitable combination of glass materials having different dispersion properties generally enables reducing the chromatic aberration. For example, in an objective lens of a telescope or the like, the chromatic aberration emerging on an axis is corrected by using a combination of a glass material having a small dispersion as a positive lens and a glass material having a large dispersion as a negative lens. However, in the case where the configuration and the number of lenses are limited, in the case where the glass material to be used for lenses is limited, and in other cases, a sufficient correction of the chromatic aberration is very difficult in some cases. One of methods to solve such a problem is a method of utilizing a glass material having an anomalous dispersion property, and designing optical elements utilizing this method is being carried out.
In the case where an optical element is manufactured which is excellent in the chromatic aberration correction function and whose shape is an aspherical shape or the like, the molding or the like of an organic resin on a spherical glass or the like has a higher advantage in being excellent in the mass-productivity, moldability, versatility and weight reduction than the use of a glass material as a material for the manufacture. However, the optical property of conventional organic resins falls in a certain range as shown in FIG. 1 (secondary dispersion property (θg,F) is 0.700 or less), and few organic resins exhibit a peculiar dispersion property.
Since many of organic substances exhibiting a peculiar dispersion property construct a conjugate structure by a double bond, a benzene ring and the like, the molecular rigidity and orientation increase, and the possibility of generation of adverse effects including a rise in the melting point and an increase in the birefringence is high. In the case of molding a material having a high melting point and a material having a high birefringence due to a molecular orientation, the molding needs to be carried out after the materials are heated to melt the crystal and destroy the orientation. However, if a polymerizable compound is heated, since the reduction in the pot life is apprehended, a material is demanded which exhibits a peculiar dispersion property and can be molded by being heated as little as possible.
If a material having a high birefringence is used for an optical element of an image pickup system, since the imaging position largely shifts, a clear image is hardly obtained. Therefore, a material is demanded which exhibits a peculiar dispersion property and has a low birefringence. A recommended magnitude of the birefringence depends on optical systems to be used and the positions for incorporating the optical systems, but if the birefringence is lower than 0.001, the applicable range of the optical systems broadens.
In the above background, US2011/0288330 proposes that sulfone (meth)acrylate being an organic resin indicated as A in FIG. 1 has a higher secondary dispersion property (higher θg,F property) than general-purpose organic resin materials.
Japanese Patent Application Laid-Open No. 2011-178985 proposes the introduction of a branched molecular structure in order to reduce the birefringence.
However, although the materials proposed in US 2011/0288330 have a high θg,F value, any of the materials have a birefringence of 0.001 or more. Although the material proposed in Japanese Patent Application Laid-Open No. 2011-178985 has an effect of reducing the birefringence, the material has a low transmittance.